1. Field of the Invention
The present invention relates to a method of optically detecting the position of an object and a position detecting apparatus making use of the method. More particularly, the invention is concerned with an absolute-type encoder which applies a linear string of light beam to a position information code pattern known as gray code and reproduces the position information carried by the pattern thereby detecting the position or displacement of the object.
2. Related Background
Photoelectric encoders have been widely used as displacement measuring apparatus which measures and detects displacement, moving speed and position of various types of objects, e.g., displacement of a movable part of an industrial machine, amount of rotation and rotational position of a robot arm, rotational amount and rotation speed of a rotational parts, and so forth.
FIG. 1A shows a rotary encoder which is capable of photoelectrically detecting amount and/or speed of rotation of a rotational object, as an example of the photoelectric encoders. The rotary encoder shown in FIG. 1A has a main scale 81 composed of a disk 85 fixed to a rotary shaft 80 and having translucent and non-translucent portions which are alternatingly arranged at a regular pitch. The rotary encoder also has a fixed index scale 82 having translucent and non-translucent portions arranged at the same regular interval as that in the main scale 81. The main scale 81 and the index scale 82 are disposed between a light projecting means 83 and a light receiving means 84 which are arranged to oppose each other. This arrangement is known and generally referred to as "index scale system". In operation, the main scale 81 rotates so that an output signal synchronous with the interval between the translucent and non-translucent portions of both scales is obtained. Any change in the speed of rotation of the main scale is detected by a frequency-analysis of the thus obtained signal. Obviously, a higher resolution of detection can be obtained by reducing the pitch or interval of the translucent and non-translucent portions.
This type of rotary encoder is generally referred to as "incremental-type encoder". In operation of this rotary encoder, the period of signal corresponds to the unit angle of rotation of the main scale, so that the amount of rotation can be detected by measuring the frequency of the signal. It is also possible to detect the instant rotational position of the main scale by calculating the amount of rotation from a certain reference rotational position.
Miniaturized incremental-type encoders having high degree of resolution have been already proposed by the same applicant in the specifications of the U.S. patent application Ser. Nos. 770,753; 880,207; 883,052; 002,229; 002,228 and 018.536.
The incremental-type encoder, therefore, suffer from a disadvantage in that it loses the instant rotational position of the object once the object is rotated while the encoder is in the inoperative position due to, for example, a failure in the power supply, even if the power supply is recovered after the start of rotation of the object.
There is another type of rotary encoder which is capable of detecting the absolute value of amount of rotation, known as absolute-type encoder. This rotary encoder has, as shown in FIG. 1B, a scale disk 85 connected to a rotary shaft 80. The scale disk has translucent and non-translucent portions which are arranged on concentric circles of different radii such that different gray codes 86, i.e., different patterns of combination of the translucent and non-translucent portions, are obtained for different unit angles. A light projecting means 83 and a light receiving means 84 are arranged to oppose each other across the scale disk 85. The light projecting means has an array of light source elements which are disposed to correspond to the respective concentric circles carrying the translucent and non-translucent portions. Similarly, the light receiving means has an array of light receiving elements which are disposed to correspond to the respective concentric circles carrying the translucent and non-translucent portions. The absolute angular or rotational position of the scale disk 85 can therefore be read by the light receiving means which receive a light pattern corresponding to the gray code peculiar to the angular position. Thus, the absolute-type rotary encoder always enables the instant rotational position of the scale disk 85 to be detected whenever the power supply is available at the time of detection, even if the scale disk 85 has been rotated during suspension of supply of the power. This means that the absolute-type rotary encoder can always provide correct information concerning the amount of movement or rotation, even if the supply of power has been suspended for an unexpected reason such as power failure. For this reason, the absolute-type rotary encoder is suitably employed in industrial robots and other industrial machines.
This absolute-type rotary encoder, however, encounters a problem in that the number of bits of the code, i.e., the numbers of the translucent and non-translucent portions in each code, has to be increased when a high resolution of position detection is needed. This in turn requires that the number of concentric circles of different radii, i.e., the number of tracks, is increased, so that the size of the rotational scale is increased undesirably.
In order to overcome this problem, Japanese Patent Laid-Open Publication No. 176817/1986 discloses an absolute-type rotary encoder in which position code information is recorded in the form of combinations of a plurality of pits which are formed in each track, and a laser emitter for emitting a laser beam which is divided into a plurality of fine laser means. The fine laser beams are applied to the respective pits and the state of reflection from the pits or transmission through the pits is detected so that the code information is reproduced.
This type of encoder also encounters a problem in that an impractically high degree of precision is required in adjusting an optical element such as a diffraction lattice for dividing the laser beam into fine beams, as well as a complicated tracking mechanism, in order to ensure that the fine laser beams are correctly directed to the pits on the tracks.